The present invention relates to a power supply unit used for electronic apparatus such as office automation equipment of various kinds. The invention also relates to an electronic apparatus using the same.
A conventional power supply unit is shown in FIG. 12.
As shown in FIG. 12, primary winding 2p of transformer 2 is connected to D.C. power supply 1. The primary winding 2p is also connected with switching element 3 and forward controller 4 for controlling the switching element 3. The forward controller 4 has forward control circuit 4a and voltage comparator 4b. Output terminal 9 is connected to secondary winding 2s of the transformer 2 through rectifier element 5, choke coil 7, and smoothing circuit 8. Another rectifier element 6, one end of which is grounded, is connected to a point of connection between the rectifier element 5 and the choke coil 7. A structure described above is an example of so-called forward-type power supply unit. A supply of 24 volts, for instance, is obtained from the output terminal 9.
In order to obtain another output from the above-described forward-type power supply unit, chopper power supply unit 12 is connected to a point of connection between the choke coil 7 and the smoothing circuit 8 through smoothing means comprised of choke coil 10 and capacitor 11. A voltage of 5 volts, for instance, is output from output terminal 17. This chopper power supply unit comprises switching element 13, rectifier element 14, choke coil 15, smoothing circuit 16, and chopper control circuit 18.
In other words, electric power is supplied to the chopper power supply unit 12 from the point of connection between the choke coil 7 and the smoothing circuit 8 of the forward-type power supply unit through the smoothing circuit comprised of the choke coil 10 and the capacitor 11. The chopper power supply unit 12 carries outs voltage conversion by chopping and controlling current that flows into the choke coil 15 with the switching element 13. As a result, it can supply electric power of several amperes or more at 5 volts from the output terminal 17.
As described above, the conventional power supply unit requires the choke coil 15 for converting voltage, and the choke coil 10 and the capacitor 11 for smoothing the voltage. That is, in order to make voltage-current conversion of the D.C. voltage efficiently, the chopper power supply unit 12 controls a conducting period of the switching element to regulate the current flowed to the choke coil 15, and outputs the converted voltage to the output terminal 17. The choke coil 15 is needed for this purpose.
Moreover, there occurs a ripple voltage in the output terminal 9, because a switching cycle of the switching element 13 does not synchronize with the forward-type power supply. The choke coil 10 and the capacitor 11 are therefore necessary to prevent it. In order to satisfy the required characteristic, it is imperative for these choke coils 10 and 15 to be considerably large in size. This is one of the reasons that make difficult to downsize and lighten the power supply unit and electronic apparatuses using the same.
The present invention is devised in consideration of the foregoing aspect, and it aims to reduce size and weight of power supply unit and electronic apparatus.
To achieve the above object, a power supply unit of this invention comprises:
a transformer having a primary winding connected in series to a D.C. power supply;
a first switching element having a control terminal and connected in the series to the primary winding;
a first rectifier element connected in series to a secondary winding of the transformer;
a choke coil connected in series to the first rectifier element;
a second switching element connected in series to the choke coil;
a first smoothing circuit connected in series to the second switching element;
a control circuit being input with an output voltage of the first smoothing circuit, for controlling the first switching element;
a second rectifier element connected in parallel with the first rectifier element and the secondary winding;
a third switching element connected in series to the choke coil;
a second smoothing circuit connected in series to the third switching element; and
a divider control circuit having a first input terminal being input with any of an input voltage and an output voltage of the second smoothing circuit, for controlling a conducting period of the third switching element responsive to the voltage being input.
An electronic apparatus of this invention comprises the power supply unit described above, a load device (e.g. an actuator), and a controller for the load device, wherein the first smoothing circuit supplies electric power to the load device, and the second smoothing circuit supplies electric power to the controller for controlling the load device.